The reaction of an alkali metal and silicon tetrafluoride to form elemental silicon has been described in U.S. Pat. No. 4,442,082 and in Final Report "Novel Duplex Vapor--Electrochemical Method for Silicon Solar Cells," March 1980, both of which are incorporated by reference. Generally the reaction may be carried out in a suitable reactor by directly contacting silicon tetrafluoride with an alkali metal such as sodium. In such a reaction, the reaction product consists essentially of a mixture of sodium fluoride and silicon. This mixture may be separated by aqueous leaching using acidic solutions to neutralize any unreacted sodium and to decrease the spontaneous oxidation of silicon that occurs rapidly in alkaline solutions.
It is known that the presence of soluble fluoride increases the amount of silicon that is lost by oxidation. This phenomenon is discussed in J. Electrochem. Soc. Vol. 127, No. 8, pp 1848-1851 (1980) wherein data is given which shows that the rate of oxidation of silicon, as measured by the evolution of hydrogen, is accelerated by the fluoride ion.
The applicant has discovered that the addition of an alkaline earth metal chloride to the leaching bath will substantially reduce the oxidation of the silicon and thus increases the yield of silicon by rapidly converting the soluble fluoride ion to a substantially completing insoluble form. It has been discovered that the use of this technique increases the rate of leaching and aids in the reduction of aluminum impurities in the silicon by coprecipition with the insoluble fluoride. The evolution of hydrogen gas is also suppressed which eliminates a safety hazard and avoids the formation of foam in the leaching stages that interferes with filtration. The cost of recovery of the silicon is also reduced because (1) the sodium fluoride in the reaction mixture dissolves more rapidly, (2) less silicon is oxidized, and (3) fewer leaching steps are required. Accordingly, it is a primary object of this invention to provide an improved method for the recovery of silicon from a reaction mixture comprising silicon and an alkali metal fluoride.
It is also an object of this invention to provide an improved method for the recovery of silicon from a reaction mixture comprising silicon and alkali metal fluoride that reduces the loss of silicon caused by oxidation.
These and other objects of the invention will become apparent from a review of the present specification.